This invention relates generally to non-desctructive testing, relates more specifically to medical diagnostic apparatus and methodology; and yet more specifically, relates to X-ray scanning apparatus and methodology of the type associated with computed tomography.
Within recent years much interest has been evidenced on the part of medical diagnosticians in the field now widely known as "computed tomography" sometimes referred to hereinafter as "CT". In a typical procedure an X-ray source and detector apparatus are positioned on opposite sides of the portion of the patient which is intended for examination. In early prior art these paired elements are made to transit across the body portion to be examined while the detectors measure the X-ray absorption at the plurality of transmission paths defined during the transit process. Periodically as well, the paired source and detector means are rotated to a different angular orientation about the body and the transit process repeated. A very high number of absorption values may be yielded by procedures of this type and the relatively massive amounts of data thus accumulated are processed by a digital computer which correlates the absorption values, to thereby derive absorption values for a very high number of points (typically in the thousands) within the section of the body being scanned.
This point-by-point data can then be combined to enable reconstruction of a matrix (visual or otherwise), which constitutes an accurate depiction of the density function of the bodily section examined. The skilled diagnostician by considering one or more of such sections can often diagnose various bodily ailments such as tumors, blood clots, etc., which features would be difficult if not impossible to effectively locate by prior art methodology.
Later developments in the computer tomography field are demonstrated in the co-pending application of John M. Pavkovich and Craig S. Nunan, Ser. No. 643,894, filed Dec. 23, 1975, and entitled "Tomographic Apparatus and Method for Reconstructing Planar Slices from Non-Absorbed Radiation" and in the co-pending application of John M. Pavkovich entitled "Apparatus and Method for Reconstructing Data", filed Dec. 23, 1975 under Ser. No. 643,896. Both of these applications are assigned to the same assignee as is the present application.
The apparatus disclosed in the above-cited applications utilizes a fan beam source of radiation coupled with application of a convolution method of image reconstruction with no intervening reordering of fan rays, to thereby eliminate the errors and delay in computation time which would otherwise be involved in such reordering. The radiation source and detector means are positioned on opposed sides of the portion of the patient being examined and these elements are made to rotate through a revolution or portion thereof while the detectors measure the radiation absorption at the plurality of transmission paths defined during the rotational process.
In a typical apparatus embodiment of the Pavkovich and Pavkovich et al type of apparatus, an assembly is provided, which is rotatable about an axis extending along a central opening defined therein, together with means for positioning the bodily portion to be examined within the central opening so that the axis of assembly rotation is perpendicular to a thin generally planar section of the body portion being scanned. A source of penetrating radiation, i.e., of X-rays or gamma rays is mounted on the assembly toward one side thereof, and provides radiation in the form of a fan beam. Means are provided for rotating the assembly so that the fan beam impinges upon the bodily portion at a plurality of incident directions. Detection is enabled by means positioned on the assembly opposite the source, which thus detects non-absorbed radiation proceeding laterally along the section.
Reference may further be usefully had to U.S. Pat. No. 4,149,249 which issued on Apr. 10, 1979 to the aforementioned John M. Pavkovich, which patent is assigned to the assignee of the present application. The said patent entitled "Apparatus and Method for Reconstructing Data" further illustrates the apparatus and methodology to which the present invention is directly applicable, and the disclosure of said patent is incorporated herein by reference.
In the U.S. Pat. No. 4,149,249, apparatus of the general type previously described is set forth, together with reconstruction means which are coupled to the detector means, and which may comprise a general purpose computer, a special purpose computer, and control logic for interfacing between these computers and controlling the respective functions thereof for permitting a convolution and back projection based upon non-absorbed and non-scattered radiation detected by the detector means. Display means are coupled to the reconstruction means for providing a visual or other display or representation of the quantities of radiation absorbed at the points considered in the object.
If the course of obtaining the detector data from which the image reconstruction is effected, it has been usual practice to effect the aforementioned relative rotation of the source detector array with respect to the patient over a time period of approximately 1 to 15 seconds (with 3 seconds being typical) while taking readings of absorbed radiation received by the detector means. The latter may be of the types disclosed in the above references. The detector means thus typically comprises of the order of 301 individual detector cells or elements which are effectively in side-by-side relationship. The source may be operated continuously, but is more often operated in pulsed fashion, and typically a set of measurements is taken at each successive 1.degree. increment of rotation (preferably by pulsing the source on at each said 1.degree. position), so that 360.times.301 values of measured (transmitted) radiation are obtained during each 360.degree. cycle of rotation.
In order to increase the available resolution possible from processing of the collected detector data, it would in principle be highly desirable to increase the number of detector elements positioned to detect the fan beam. In practice, however, both size and cost limitations have rendered this relatively impractical.
For various reasons it would further be desirable in operation of a computerized tomography system to be able to effect data collection during less than a full revolution of the gantry or platform carrying the source and detector elements undergoing planetary motion about the scanned object. Such a procedure would be desirable, partially in order to shorten the period during which the patient is required to remain motionless; and additionally, to reduce the total incident radiation to which the patient is exposed. In the past, however, efforts to operate in the aforementioned manner have not been successful since measuring projections are missing, which in the past have either been arbitrarily estimated or have been taken to be zero. These assumptions have lead to undesirable artifacts and loss of definition in the resultant reconstructed image.
It is in principle also of interest to the technician in the present field of application, to be able to study dynamic events as, for example, in performing cardiac studies, where one is often interested in examining a section through portions of the heart at selected phases of the cardiac cycle.
Similarly, it is of interest to study other medically significant dynamic events such as blood flow, which may have extreme value in diagnosing aortical aneurisms, infarctions, or the like. In the co-pending application of Edward J. Seppi et al., Ser. No. 789,910, filed Apr. 22, 1977 and entitled "Method and System for Cardiac Computed Tomography", which again is assigned to the assignee of the present invention, a method and system is disclosed which is applicable to the aforementioned type of studies. In this system data is acquired during one or more full rotational cycles and suitably stored. The data corresponding to various angular projections can then be correlated with the desired portions of the object's cyclical motion by means of a reference signal associated with the motion, such as that derived through an electrocardiagram where a heart is the object of interest. In systems and approaches of the types as mentioned, however, projections are invariably missing, i.e., a full 360.degree. collection of projections are not usually present; and partially for this reason the image resolution and quality, again as aforementioned, can be quite unsatisfactory.
In accordance with the foregoing, it may be regarded as an object of the present invention to provide a method and system applicable to computerized tomography systems of the type heretofore discussed, which enables an increase in available resolution and image quality possible from processing of the collected detector data, by effectively increasing the number of detectors positioned to detect the fan beam.